1. Field of the Invention
This invention concerns wind speed measurement, and relates in particular to the detection and/or measurement of wind gradient.
Wind is the large-scale movement of air from one place to another. Though for most purposes it may be convenient to visualize this movement as being that of a block of air, the air in any one part of the block being stationary with respect to the air in any other part so that all parts of the block move over the ground with the same velocity, in fact this is generally not the case. Instead, it is usual--especially in the case of wind blowing across large open spaces--for the air to become horizontally stratified; indeed, it may be regarded as a series of layers parallel to the ground and moving at different velocities, the layers nearer the ground moving more slowly (because of friction or viscosity effects) than those higher up. Thus, a 30 knot wind moving across an airfield (say) would truly have a speed of 30 knots at a height of 50 feet and above, but at 20 feet its speed might only be 25 knots, at 10 feet 20 knots--and close to ground level the air speed might only be 10 knots. This change of wind speed with height is known as wind gradient (or wind shear), and it may cause serious problems for aircraft during their approach and landing.
As is well known, all aircraft deriving their lift purely by virtue of the passage of air across their wings have a critical angle of attack (the angle the wing chord makes with the direction of motion) beyond which the wing lift produced suddenly and disastrously decreases by a large amount. The resultant rather precipitous tendency of the aircraft to fall out of the sky is known as a stall, and the angle of attack at which the stall occurs is referred to as the stall angle. The stall angle is in fact more or less independent of flying speed, but in general stalling occurs most commonly at a very low flying speed, which speed is thus referred to (inaccurately, though understandably) as the stalling speed. Both the stall angle and the stalling speed vary with aircraft type; a glider trainer may have a stalling speed as low as 30 knots, while a modern light aircraft may stall at 55 knots. A Jumbo Jet may have a stalling speed as high as 100 knots.
As stated above, a conventional aircraft is kept up by the lift resulting from air moving across its wings, and this lift is directly related to the speed of this air movement. Hence, a satisfactory guide to the aircraft's lift situation is its speed through the air--its airspeed. In most cases the relation of an aircraft's airspeed to its ground speed (its speed over the ground) is unimportant as regards its ability to keep flying. At the moment of landing, however, the situation is different.
It is general practice to land an aircraft by flying it down to just above ground level at an airspeed slightly above stalling speed, and then to allow the airspeed to drop slowly until the aircraft sinks onto the runway. Ideally the airspeed should then drop below stalling speed to prevent the aircraft bouncing back into the air. If the aircraft tries to land with too high an airspeed it keeps on flying--at ground level--any may reach the end of the runway before it can actually land, slow down, and stop. If, on the other hand, it tries to land with too low an airspeed it may stall onto the runway from a significant height. A Jumbo Jet may have a stalling speed of 100 knots but a landing speed of only 110 knots; it can easily be appreciated that even a slight error in the airspeed of a Jumbo on its landing approach may have disastrous results. Unfortunately, it is at this critical moment that wind gradient effects can exacerbate the situation. Taking the case of the Jumbo referred to above landing through a head wind with the gradient also referred to above, it can easily be seen that an airspeed at 50 feet of 110 knots (into a 50 ft. level head wind of 30 knots this gives a true ground speed of 110-30=80 knots) becomes--because of the wind gradient change of wind speed with height--a near ground level airspeed of 90 knots (the true ground speed of 80 knots plus the near ground level headwind of 10 knots). This is 10 knots below the aircraft's stalling speed, and the result is somewhat unfortunate! Naturally, the answer is to fly faster on the approach when landing through a wind gradient. The difficulty here, however, is that an extra 10 to 15 knots airspeed on the approach to allow for a wind gradient that may not after all exist could mean running off the end of the runway--which would also be somewhat unfortunate.
2. Description of the Prior Art
The problem can only really be solved by ascertaining the wind gradient situation, but at the moment there is no satisfactory way of doing this. It might be thought that wind speed could be measured at different heights directly using a series of vertically-separated anemometers of the standard drag-cup type, but in fact these instruments are incapable of giving a result of the required accuracy and reliability, and it is common to present to "measure" wind gradient on the highly unsatisfactory basis of reports from the pilots of aircraft that have already landed. The present invention seeks to provide acceptable apparatus for, and a method of, actually detecting--and preferably measuring--wind gradient, and is based upon the apparent speed of sound in air between two fixed points being affected by the motion of the air relative to the points.